Fecth&UpdateData on ListGrid not executed

Hello,

I am using SmartClient/SmartGWT Framework v9.1p_2014-04-18/Pro Deployment.

I created an array of two listgrids which are shown in tabs of a tabset and which are associated with the same datasource.
If fetch is called on listgrid[0] it is not executed. The developer console does not show an RPC-call.
If I change list[0].fetch to list[0].getDataSource.fetch the fetch is executed.
Same holds for updateData

Where's my mistake?

Thank you
Andreas